1. Field of the Invention
The present invention relates to a lens barrel having at least two joinable cylinders.
2. Related Background Art
A prior art lens barrel of this type is explained below.
FIG. 1 shows a sectional view of a compact camera, in which a male helicoid 3 meshes with an inside of a female helicoid 1 which is rotated around a center of an optical axis by a motor, not shown and a gear train, not shown. The male helicoid 3 is made of plastic and driven along the optical axis by a straight key 2. A cylinder 4 having a lens barrier, not shown is joined as shown by 5 in FIG. 1. The cylinder 4 is also made of plastic, and the lens barrel is formed by joining the male helicoid 3 and the cylinder 4 together.
The lens barrel should be joint free and integral, but in order to allow mounting of the lens barrier and a lens shutter in the lens barrel because of the internal structure of the lens barrel, one portion (the cylinder 4) and another portion (the male helicoid 3) are provided, and the lens barrier is arranged in the one portion while the lens shutter is arranged in the other portion, and they are joined to form the lens barrel.
An enlarged view of the joint 5 of the male helicoid 3 and the cylinder 4 is shown in FIG. 2. In the joint, a periphery 4a is formed on an inner periphery of the cylinder 4 and a periphery 3a is formed on an outer periphery of the male helicoid 3, and the periphery 3a is fitted to the periphery 4a of the cylinder 4 so that the male helicoid 3 and the cylinder 4 are joined as shown by the joint 5 of FIG. 1.
FIG. 3 shows an enlarged view of another prior art joint. Tapers 6a and 7a are formed in a male helicoid 6 and a cylinder 7, and the male helicoid 6 is joined to the cylinder 7 while the tapers 6a and 7a face to each other to form a lens barrel.
The tapers 6a and 7a are provided to prevent the occurrence of a "sink mark" which is a disadvantage in the plastic and defined as follows. When a molding material in a mold is cooled to solidify, a thinner portion of the molded article is solidified first. As a result, plastic in a thicker area is pulled by the thinner portion so that a recess is formed in the thicker area. The recess created on the surface of the molded article is called sink mark.
In FIG. 2, since abutments 3a and 4a of the male helicoid 3 and the cylinder 4 are planar as shown, a leakage of light into the lens barrel does not occur even if the joint is loose and a gap a is created as shown, but the following problem arises because the portion 4c which includes the abutment 4a is thin and a portion 4e on the left of a root 4d of the portion 4c is thick.
The right hand thin portion 4c is solidified first and a sink mark is created in the left hand thick portion 4e by the portion 4c so that the portion 4e is deformed and the root 4d of the thin portion 4c is also deformed. By this deformation, the right hand thin portion 4c is deformed such that it is torn off the abutment 3a of the male helicoid 3 to create a gap between it and the abutment 3a of the male helicoid 3, and the leakage of light occurs from the exterior of the lens barrel to the interior through the gap.
As the thin portion 4c of the cylinder 4 is formed to be torn off the abutment 3a of the male helicoid 3, the external appearance of the lens barrel is spoiled. Further, the right hand thin portion 4c may be bent toward the inner hollow area of the cylinder 4 by the sink mark of the thick portion 4e before the cylinder 4 is joined to the male helicoid 3. As a result, the cylinder 4 is hard to be joined to the male helicoid 3.
A sink mark is also produced in the thick portion 3e of the male helicoid 3 by the solidification of the thin portion 3c including the abutment 3a through the root 3d, and the thin portion 3c is deformed to be bent by the sink mark. As a result, a gap is created, which causes the leakage of light and deteriorates the joint to the cylinder 4.
In order to solve the problem of the sink mark, the material may be changed from plastic to metal but it raises a problem of increased cost.
In FIG. 3, in order to prevent the sink mark, tapers 6a and 7a are formed. Specifically, the thickness of a portion 7c on the right of a root of a joint 7b of the cylinder is not much thinner than a thick portion 7e on the left as it is at an end portion 7d but it is slightly thinner than the thick portion 7e. As a result, the sink mark is hard to form in the thick portion 7e. Further, by the formation of the taper 7a, the thickness of the joint 7b is gradually thinner as it goes from the left to the right so that the sink mark is hard to form in the joint 7b. As a result, the joint is not deformed and the problem discussed above in connection with FIG. 2 may be solved. Since the taper 6a is formed in the male helicoid as well, the above problem may be solved.
However, because of the tapers 6a and 7a, the left cylinder 7 is retracted from the male helicoid 6 so that a gap a shown in FIG. 3 and a gap between the tapers 6a and 7a are created and the leakage of light takes place as described above. In order to prevent the leakage of light, the joint should be made with care but it impedes the assembling work and leads to increased cost.
When an attempt is made to provide a light shielding member in the lens barrel in order to prevent the leakage of light, the construction is complex, the number of parts increases, and the cost rises. Since the inner diameter of the lens barrel is determined by the internal members such as a shutter, the size of the lens barrel increases when the light shielding member is provided. This is the opposite of the intended goal of compactness of the lens barrel.